Soft tissue

ABSTRACT

Soft throughdried tissues, which are sufficiently soft to serve as premium bathroom tissues, can be made without the use of a Yankee dryer. The typical Yankee functions of building machine direction and cross-machine direction stretch are replaced by a wet end rush transfer and the throughdrying fabric design, respectively. It is particularly advantageous to form the tissue with chemimechanically treated fibers in at least one layer. The resulting tissues have high bulk (about 6 cubic centimeters per gram or greater) and low stiffness.

This application is a continuation of application Ser. No. 08/815,407entitled “SOFT TISSUE” and filed in the U.S. Patent and Trademark Officeon Mar. 10, 1997, now U.S. Pat. No. 5,932,068 which is a continuation ofapplication Ser. No. 08/399,277 entitled “SOFT TISSUE” and filed in theU.S. Patent and Trademark Office on Mar. 6, 1995, now U.S. Pat. No.5,656,132, which is a divisional of application Ser. No. 08/082,684,filed on Jun. 24, 1993, now U.S. Pat. No. 5,607,551. The entirety ofthis application is hereby incorporated by reference.

BACKGROUND OF THE INVENTION

In the manufacture of tissue products such as bath tissue, a widevariety of product characteristics must be given attention in order toprovide a final product with the appropriate blend of attributessuitable for the product's intended purposes. Among these variousattributes, improving softness has always been a major objective forpremium products. Major components of softness include stiffness andbulk (density), with lower stiffness and higher bulk (lower density)generally improving perceived softness.

Traditionally, tissue products have been made using a wet-pressingprocess in which a significant amount of water is removed from a wetlaid web by pressing or squeezing water from the web prior to finaldrying. In particular, while supported by an absorbent papermaking felt,the web is squeezed between the felt and the surface of a rotatingheated cylinder (Yankee dryer) using a pressure roll as the web istransferred to the surface of the Yankee dryer for final drying. Thedried web is thereafter dislodged from the Yankee dryer with a doctorblade (creping), which serves to partially debond the dried web bybreaking many of the bonds previously formed during the wet-pressingstages of the process. Creping generally improves the softness of theweb, albeit at the expense of a significant loss in strength.

More recently, throughdrying has become a more prevalent means of dryingtissue webs. Throughdrying provides a relatively noncompressive methodof removing water from the web by passing hot air through the web untilit is dry. More specifically, a wet-laid web is transferred from theforming fabric to a coarse, highly permeable throughdrying fabric andretained on the throughdrying fabric until it is dry. The resultingdried web is softer and bulkier than a wet-pressed uncreped dried sheetbecause fewer papermaking bonds are formed and because the web is lessdense. Squeezing water from the wet web is eliminated, althoughsubsequent transfer of the web to a Yankee dryer for creping is stillused to final dry and/or soften the resulting tissue.

While there is a processing incentive to eliminate the Yankee dryer andmake an uncreped throughdried tissue, attempts to make throughdriedtissue sheets without using a Yankee dryer (uncreped) have heretoforelacked adequate softness when compared to their creped counterparts.This is partially due to the inherently high stiffness and strength ofan uncreped sheet, since without creping there is no mechanicaldebonding in the process. Because stiffness is a major component ofsoftness, the use of uncreped throughdried sheets has been limited toapplications and markets where high strength is paramount, such as forindustrial wipers and towels, rather than for applications wheresoftness is required, such as for bath tissue, premium household towels,and facial tissue in the consumer market.

SUMMARY OF THE INVENTION

It has now been discovered that tissues having properties particularlysuitable for use as a bath tissue can be made using certain pretreatedpapermaking fibers in an appropriate process. A throughdrying tissuemaking process in which the tissue web is not adhered to a Yankee dryerand hence is uncreped is preferred. The resulting tissues of thisinvention are characterized by a unique combination of high bulk and lowstiffness as compared to available creped bath tissue products andespecially so as compared to prior uncreped throughdried products.

The stiffness of the products of this invention can be objectivelyrepresented by either the maximum slope of the machine direction (MD)load/elongation curve for the tissue (hereinafter referred to as the “MDMax Slope”) or by the machine direction Stiffness Factor (hereinafterdefined), which further takes into account the caliper of the tissue andthe number of plies of the product. In accordance with this invention,by overcoming the inherently high stiffness of uncreped throughdriedsheets, an acceptably soft tissue with high bulk and low stiffness canbe produced. In addition, the products of this invention can have a highdegree of stretch of about 10 percent or greater, which provides in-usedurability. Such soft, strong and stretchable tissue products with highbulk have heretofore never been made. While this invention isparticularly applicable to bath tissue, it is also useful for otherpaper products where softness is a significant attribute, such as forfacial tissue and household paper towels.

Hence in one aspect, the invention resides in a soft tissue having aBulk (hereinafter defined) of about 9 cubic centimeters per gram orgreater and an MD Max Slope of about 10 or less.

In another aspect, the invention resides in an a soft tissue comprisingone or more uncreped throughdried plies and having a MD Max Slope ofabout 10 or less, preferably also having a Bulk of about 6 cubiccentimeters per gram or greater.

In another aspect, the invention resides in a soft tissue having a Bulkof about 9 cubic centimeters per gram or greater and a MD StiffnessFactor of about 150 or less.

In another aspect, the invention resides in a soft tissue comprising oneor more uncreped throughdried plies and having a MD Stiffness Factor ofabout 150 or less, preferably also having a Bulk of about 6 cubiccentimeters per gram or greater.

In a further aspect, the invention resides in a method of making a softtissue sheet comprising: (a) forming an aqueous suspension ofpapermaking fibers having a consistency of about 20 percent or greater;(b) mechanically working the aqueous suspension at a temperature of 140°F. or greater provided by an external heat source, such as steam, with apower input of about 1 horsepower-day per ton of dry fiber or greater tocurl the fibers; (c) diluting the aqueous suspension of curled fibers toa consistency of about 0.5 percent or less and feeding the dilutedsuspension to a tissue-making headbox; (d) depositing the dilutedaqueous suspension onto a forming fabric to form a wet web; (e)dewatering the wet web to a consistency of from about 20 to about 30percent; (f) transferring the dewatered web from the forming fabric to atransfer fabric traveling at a speed of from about 10 to about 80percent slower than the forming fabric; (g) transferring the web to athroughdrying fabric whereby the web is macroscopically rearranged toconform to the surface of the throughdrying fabric; and (h)throughdrying the web to final dryness.

The Bulk of the products of this invention is calculated as the quotientof the Caliper (hereinafter defined), expressed in microns, divided bythe basis weight, expressed in grams per square meter. The resultingBulk is expressed as cubic centimeters per gram. For the products ofthis invention, Bulks can be about 6 cubic centimeters per gram orgreater, preferably about 9 cubic centimeters per gram or greater,suitably from about 9 to about 20 cubic centimeters per gram, and morespecifically from about 10 to about 15 cubic centimeters per gram. Theproducts of this invention derive the Bulks referred to above from thebasesheet, which is the sheet produced by the tissue machine withoutpost treatments such as embossing. Nevertheless, the basesheets of thisinvention can be embossed to produce even greater bulk or aesthetics, ifdesired, or they can remain unembossed. In addition, the basesheets ofthis invention can be calendered to improve smoothness or decrease theBulk if desired or necessary to meet existing product specifications.

The MD Max Slope of the products of this invention can be about 10 orless, preferably about 5 or less, and suitably from about 3 to about 6.Determining the MD Max Slope will be hereinafter described in connectionwith FIG. 6. The MD Max Slope is the maximum slope of the machinedirection load/elongation curve for the tissue. The units for the MD MaxSlope are kilograms per 3 inches (7.62 centimeters), but for conveniencethe MD Max Slope values are hereinafter referred to without the units.

The MD Stiffness Factor of the products of this invention can be about150 or less, preferably about 100 or less, and suitably from about 50 toabout 100. The MD Stiffness Factor is calculated by multiplying the MDMax Slope by the square root of the quotient of the Caliper divided bythe number of plies. The units of the MD Stiffness Factor are (kilogramsper 3 inches)-microns^(0.5), but for simplicity the values of the MDStiffness Factor are hereinafter referred to without the units.

The Caliper as used herein is the thickness of a single sheet, butmeasured as the thickness of a stack of ten sheets and dividing the tensheet thickness by ten, where each sheet within the stack is placed withthe same side up. Caliper is expressed in microns. It is measured inaccordance with TAPPI test methods T402 “Standard Conditioning andTesting Atmosphere For Paper, Board, Pulp Handsheets and RelatedProducts” and T411 om-89 “Thickness (caliper) of Paper, Paperboard, andCombined Board” with Note 3 for stacked sheets. The micrometer used forcarrying out T411 om-89 is a Bulk Micrometer (TMI Model 49-72-00,Amityville, N.Y.) having an anvil diameter of 4{fraction (1/16)} inches(103.2 millimeters) and an anvil pressure of 220 grams/square inch (3.39kiloPascals). After the Caliper is measured, the same ten sheets in thestack are used to determine the average basis weight of the sheets.

The products of this invention can be single-ply products or multi-plyproducts, such as two-ply, three-ply, four-ply or greater. One-plyproducts are advantageous because of their lower cost of manufacture,while multi-ply products are preferred by many consumers. For multi-plyproducts it is not necessary that all plies of the product be the same,provided at least one ply is in accordance with this invention.

The basis weight of the products of this invention can be from about 5to about 70 grams per square meter (gsm), preferably from about 10 toabout 40 gsm, and more preferably from about 20 to about 30 gsm. For asingle-ply bath tissue, a basis weight of about 25 gsm is preferred. Fora two-ply tissue, a basis weight of about 20 gsm per ply is preferred.For a three-ply tissue, a basis weight of about 15 gsm per ply ispreferred.

The tissues of this invention can also be characterized by a relativelyhigh degree of machine direction stretch. The amount of machinedirection stretch can be about 10 percent or greater, suitably fromabout 15 to about 25 or 30 percent. Cross-machine direction (CD) stretchcan be about 3 percent or greater, suitably from about 7 to about 10percent. Machine direction stretch can be imparted to the sheet upontransfer of the web from the forming fabric to the transfer fabric,and/or by transfer from a transfer fabric to another transfer fabric,and/or by transfer of the web from a transfer fabric to thethroughdrying fabric. Cross-machine direction stretch is dominated bythe throughdrying fabric design.

In order to be suitable for use as a bath tissue, the machine directiontensile strength is preferably about 600 grams per 3 inches (7.62centimeters) of width or greater, more suitably from about 700 to about1500 grams. Cross-machine direction tensile strengths are preferablyabout 300 grams per 3 inches (7.62 centimeters) of width or greater,more suitably from about 400 to about 600 grams.

The MD Tensile Strength, MD Tensile Stretch, CD Tensile Strength and CDTensile Stretch are obtained according to TAPPI Test Method 494 OM-88“Tensile Breaking Properties of Paper and Paperboard” using thefollowing parameters: Crosshead speed is 10.0 in/min. (254 mm/min), fullscale load is 10 lb (4,540 g), jaw span (the distance between the jaws,sometimes referred to as the gauge length) is 2.0 inches (50.8 mm),specimen width is 3 inches (76.2 mm). The tensile testing machine is aSintech, Model CITS-2000 (Systems Integration Technology Inc.,Stoughton, Mass.; a division of MTS Systems Corporation, ResearchTriangle Park, N.C.).

Papermaking fibers useful for purposes of this invention include anycellulosic fibers which are known to be useful for making paper,particularly those fibers useful for making relatively low densitypapers such as facial tissue, bath tissue, paper towels, dinner napkinsand the like. Suitable fibers include virgin softwood and hardwoodfibers, as well as secondary or recycled cellulosic fibers, and mixturesthereof. Especially suitable hardwood fibers include eucalyptus andmaple fibers. As used herein, “secondary fiber” means any cellulosicfiber which has previously been isolated from its original matrix viaphysical, chemical or mechanical means and, further, has been formedinto a fiber web, dried to a moisture content of about 10 weight percentor less and subsequently reisolated from its web matrix by somephysical, chemical or mechanical means.

A key component in tissue softness is sheet stiffness or resistance tofolding. Previous processes decrease stiffness via creping, layering,patterned attachment to the Yankee dryer or some combination of these.Neither the first nor last process is possible in an uncrepedthroughdried process. Therefore, layering is expected to play a key rolein reducing sheet stiffness at the required overall tensile strength.Ideally, the desired overall strength would be carried in a very thinlayer (for low stiffness) which has been treated to give very highstrength or modulus (perhaps by refining or chemical action). Theremaining layer(s) would comprise fibers which have been treated tosignificantly reduce their strength (modulus). The key to achieving lowstiffness at required overall strength then becomes treating ormodifying the fibers in such a way as to maximize the difference instrength (modulus) of the layers. An ideal modification for the weakerlayer would simultaneously reduce tensile strength and increase bulk, asthis would decrease modulus the greatest.

The modification methods to produce soft fibers for the relatively weaklayers include mechanical modification, chemical modification andcombinations of mechanical modification and chemical modification.Mechanical modifications are achieved by methods which permanentlydeform the fibers through mechanical action. These methods introducecurl, kinks, and microcompressions into the fiber which decreasefiber-to-fiber bonding, decrease sheet tensile strength, and increasesheet bulk, stretch, porosity and softness. Examples of suitablemechanical modification methods include flash drying, dry fiberizing andwet high-consistency curling. While any process or mechanical devicewhich imparts fiber curl may increase sheet softness, those whichproduce more curl or a stiffer curl or a more permanent curl uponexposure to water will increase sheet softness to a greater extent andare hence preferred. In addition, softness-enhancing chemicals can beadded to mechanically-modified fibers either before or after mechanicalmodification to produce further increases in softness over themechanical treatment or wet end chemical addition alone. A preferredmeans for modifying the fibers for purposes of this invention is to passthe fibers through a shaft disperser, which is a wet high-consistencycurling device which works the fibers (imparts high shear forces and ahigh degree of inter-fiber friction) at elevated temperature. Fiberswhich have been passed through a shaft disperser (sometimes referred toherein as “dispersing”) are referred to as “dispersed fibers”. Thesefibers possess certain properties which make them particularlyadvantageous for making uncreped throughdried tissues because of theirbulk building ability and their softness.

The consistency of the aqueous fiber suspension which is subjected tothe dispersing treatment must be high enough to provide significantfiber-to-fiber contact or working which will alter the surfaceproperties of the treated fibers. Specifically, the consistency can beat least about 20, more preferably from about 20 to about 60, and mostpreferably from about 30 to about 50 dry weight percent. The consistencywill be primarily dictated by the kind of machine used to treat thefibers. For some rotating shaft dispersers, for example, there is a riskof plugging the machine at consistencies above about 40 dry weightpercent. For other types of dispersers, such as the Bivis machine(commercially available from Clextral Company, Firminy Cedex, France),consistencies greater than 50 can be utilized without plugging. Thisdevice can be generally described as a pressurized twin screw shaftdisperser, each shaft having several screw flights oriented in thedirection of material flow followed by several flights oriented in theopposite direction to create back pressure. The screw flights arenotched to permit the material to pass through the notches from oneseries of flights to another. It is desirable to utilize a consistencywhich is as high as possible for the particular machine used in order tomaximize fiber-to-fiber contact.

The temperature of the fibrous suspension during dispersing can be about140° F. or greater, preferably about 150° F. or greater, more preferablyabout 210° F. or greater, and most preferably about 220° F. or greater.The upper limit on the temperature is dictated by whether or not theapparatus is pressurized, since the aqueous fibrous suspensions withinan apparatus operating at atmospheric pressure cannot be heated beyondthe boiling point of water. Interestingly, it is believed that thedegree and permanency of the curl is greatly impacted by the amount oflignin in the fibers being subjected to the dispersing process, withgreater effects being attainable for fibers having higher lignincontent. Hence high yield pulps having a high lignin content areparticularly advantageous in that fibers previously considered notsuitably soft can be transformed into suitably soft fibers. Such highyield pulps, listed in decreasing order of lignin content, aregroundwood, thermomechanical pulp (TMP), chemimechanical pulp (CMP), andbleached chemithermomechanical pulp (BCTMP). These pulps have lignincontents of about 15 percent or greater, whereas chemical pulps (kraftand sulfite) are low yield pulps having a lignin content of about 5percent or less.

The amount of power applied to the fibrous suspension during dispersingalso impacts the fiber properties. In general, increasing the powerinput will increase the fiber curl. However, it has also been found thatthe fiber curl reaches a maximum upon reaching a power input of about 2horsepower-days per ton (HPD/T) (1.6 kilowatt-days per tonne) of dryfiber in suspension. A preferred range of power input is from about 1 toabout 3 HPD/T (0.8 to about 2.5 kilowatt-days per tonne), morepreferably about 2 HPD/T (1.6 kilowatt-days per tonne) or greater.

In working the fibers during dispersing, it is necessary that the fibersexperience substantial fiber-to-fiber rubbing or shearing as well asrubbing or shearing contact with the surfaces of the mechanical devicesused to treat the fibers. Some compression, which means pressing thefibers into themselves, is also desirable to enhance or magnify theeffect of the rubbing or shearing of the fibers. The measure of theappropriate amount of shearing and compression to be used lies in theend result, which is the achievement of high bulk and low stiffness inthe resulting tissue. A number of shaft dispersers or equivalentmechanical devices known in the papermaking industry can be used toachieve varying degrees of the desired results. Suitable shaftdispergers include, without limitation, nonpressurized shaft dispergersand pressurized shaft dispersers such as the Bivis machines describedabove. Shaft dispersers can be characterized by their relatively highvolume:internal surface area ratio and rely primarily on fiber-to-fibercontact to cause fiber modification. This is in contrast with discrefiners or disc dispergers, which rely primarily on metalsurface-to-fiber contact rather than fiber-to-fiber contact. Whiledispersing is a preferred method of modulus reduction for soft layerfibers, it is not intended that this invention be limited by the use offibers treated in this manner. Mechanical or chemical means can be usedto decrease the strength and modulus of these fibers and employed alongwith a strength layer to directionally reduce sheet stiffness.

Softening agents, sometimes referred to as debonders, can be used toenhance the softness of the tissue product and such softening agents canbe incorporated with the fibers before, during or after disperging. Suchagents can also be sprayed or printed onto the web after formation,while wet, or added to the wet end of the tissue machine prior toformation. Suitable agents include, without limitation, fatty acids,waxes, quaternary ammonium salts, dimethyl dihydrogenated tallowammonium chloride, quaternary ammonium methyl sulfate, carboxylatedpolyethylene, cocamide diethanol amine, coco betaine, sodium laurylsarcosinate, partly ethoxylated quaternary ammonium salt, distearyldimethyl ammonium chloride, polysiloxanes and the like. Examples ofsuitable commercially available chemical softening agents include,without limitation, Berocell 596 and 584 (quaternary ammonium compounds)manufactured by Eka Nobel Inc., Adogen 442 (dimethyl dihydrogenatedtallow ammonium chloride) manufactured by Sherex Chemical Company,Quasoft 203 (quaternary ammonium salt) manufactured by Quaker ChemicalCompany, and Arquad 2HT-75 (di(hydrogenated tallow) dimethyl ammoniumchloride) manufactured by Akzo Chemical Company. Suitable amounts ofsoftening agents will vary greatly with the species selected and thedesired results. Such amounts can be, without limitation, from about0.05 to about 1 weight percent based on the weight of fiber, morespecifically from about 0.25 to about 0.75 weight percent, and stillmore specifically about 0.5 weight percent.

Referring now to the tissue making process of this invention, theforming process and tackle can be conventional as is well known in thepapermaking industry. Such formation processes include Fourdrinier, roofformers (such as suction breast roll), and gap formers (such as twinwire formers, crescent formers), etc. A twin wire former is preferredfor higher speed operation. Forming wires or fabrics can also beconventional, the finer weaves with greater fiber support beingpreferred to produce a smoother sheet and the coarser weaves providinggreater bulk. Headboxes used to deposit the fibers onto the formingfabric can be layered or nonlayered, although layered headboxes areadvantageous because the properties of the tissue can be finely tuned byaltering the composition of the various layers.

More specifically, for a single-ply product it is preferred to provide athree-layered tissue having dispersed fibers on both the “air side” ofthe tissue and on the “fabric side” of the tissue. (The “air side”refers to the side of the tissue not in contact with the fabric duringdrying, while the “fabric side” refers to the opposite side of thetissue which is in contact with the throughdryer fabric during drying.)The center of the tissue preferably comprises ordinary softwood fibersor secondary fibers, which have not been dispersed, to impart sufficientstrength to the tissue. However, it is within the scope of thisinvention to include dispersed fibers in all layers. For a two-plyproduct, it is preferred to provide dispersed fibers on the fabric sideof the tissue sheet and ply the two tissue sheets together such that thedisperged fiber layers become the outwardly facing surfaces of theproduct. Nevertheless, the dispersed fibers (virgin fibers or secondaryfibers) can be present in any or all layers depending upon the sheetproperties desired. In all cases the presence of dispersed fibers canincrease Bulk and lower stiffness. The amount of disperged fibers in anylayer can be any amount from 1 to 100 weight percent, more specificallyabout 20 weight percent or greater, about 50 weight percent or greater,or about 80 weight percent or greater. It is preferred that thedisperged fibers be treated with a debonder as herein described tofurther enhance Bulk and lower stiffness.

In manufacturing the tissues of this invention, it is preferable toinclude a transfer fabric to improve the smoothness of the sheet and/orimpart sufficient stretch. As used herein, “transfer fabric” is a fabricwhich is positioned between the forming section and the drying sectionof the web manufacturing process. The fabric can have a relativelysmooth surface contour to impart smoothness to the web, yet must haveenough texture to grab the web and maintain contact during a rushtransfer. It is preferred that the transfer of the web from the formingfabric to the transfer fabric be carried out with a “fixed-gap” transferor a “kiss” transfer in which the web is not substantially compressedbetween the two fabrics in order to preserve the caliper or bulk of thetissue and/or minimize fabric wear.

Transfer fabrics include single-layer, multi-layer or compositepermeable structures. Preferred fabrics have at least one of thefollowing characteristics: (1) On the side of the transfer fabric thatis in contact with the wet web (the top side), the number of machinedirection (MD) strands per inch (mesh) is from 10 to 200 (4 to 80 percentimeter) and the number of cross-machine direction (CD) strands perinch (count) is also from 10 to 200. The strand diameter is typicallysmaller than 0.050 inch (1.3 millimeter); and (2) on the top side, thedistance between the highest point of the MD knuckle and the highestpoint of the CD knuckle is from about 0.001 to about 0.02 or 0.03 inch(0.025 to about 0.5 or 0.75 millimeter). In between these two levels,there can be knuckles formed either by MD or CD strands that give thetopography a 3-dimensional characteristic. Specific suitable transferfabrics include, by way of example, those made by Asten Forming Fabrics,Inc., Appleton, Wis., and designated as numbers 934, 937, 939 and 959and Albany 94 M manufactured by Albany International, Appleton WireDivision, Appleton, Wis.

In order to provide stretch to the tissue, a speed differential isprovided between fabrics at one or more points of transfer of the wetweb. The speed difference between the forming fabric and the transferfabric can be from about 5 to about 75 percent or greater, preferablyfrom about 10 to about 35 percent, and more preferably from about 15 toabout 25 percent, based on the speed of the slower transfer fabric. Theoptimum speed differential will depend on a variety of factors,including the particular type of product being made. As previouslymentioned, the increase in stretch imparted to the web is proportionalto the speed differential. For a single-ply uncreped throughdried bathtissue having a basis weight of about 25 grams per square meter, forexample, a speed differential of from about 20 to about 25 percentbetween the forming fabric and a sole transfer fabric produces a stretchin the final product of from about 15 to about 25 percent. The stretchcan be imparted to the web using a single differential speed transfer ortwo or more differential speed transfers of the wet web prior to drying.Hence there can be one or more transfer fabrics. The amount of stretchimparted to the web can hence be divided among one, two, three or moredifferential speed transfers. The web is transferred to the last fabric(the throughdrying fabric) for final drying preferably with theassistance of vacuum to ensure macroscopic rearrangement of the web togive the desired Bulk and appearance. The use of separate transfer andthroughdrying fabrics offers a significant improvement over the priorart since it allows the two fabrics to be designed specifically toaddress key product requirements independently. For example, thetransfer fabrics are generally optimized to allow efficient conversionof high rush transfer levels to high MD stretch and to improve sheetsmoothness while throughdrying fabrics are designed to deliver bulk andCD stretch. It is therefore useful to have quite fine and relativelyplanar transfer fabrics and throughdrying fabrics which are quite coarseand three dimensional in the optimized configuration. The result is thata relatively smooth sheet leaves the transfer section and then ismacroscopically rearranged (with vacuum assist) to give the high bulk,high CD stretch surface topology of the throughdrying fabric. No visible(at least not macroscopically visible) trace of the transfer fabricremains in the finished product. Sheet topology is completely changedfrom transfer to throughdrying fabric and fibers are macroscopicallyrearranged, including significant fiber-fiber movement.

The drying process can be any noncompressive drying method which tendsto preserve the bulk or thickness of the wet web including, withoutlimitation, throughdrying, infra-red radiation, microwave drying, etc.Because of its commercial availability and practicality, throughdryingis well-known and is a preferred means for noncompressively drying theweb for purposes of this invention. Suitable throughdrying fabricsinclude, without limitation, Asten 920A and 937A and Velostar P800 and103A. The web is preferably dried to final dryness on the throughdryingfabric, without being pressed against the surface of a Yankee dryer, andwithout subsequent creping. This provides a product of relativelyuniform density as compared to products made by a process in which theweb was pressed against a Yankee while still wet and supported by thethroughdrying fabric or by another fabric, or as compared to spot-bondedairlaid products. Although the final product appearance and bulk aredominated by the throughdrying fabric design, the machine directionstretch in the web is primarily provided by the transfer fabric, thusgiving the method of this invention greater process flexibility.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a schematic process flow diagram illustrating a method ofmaking uncreped throughdried sheets in accordance with this invention.

FIG. 2 is a schematic process flow diagram of a method of treatingfibers in accordance with this invention using a shaft disperser to workthe fibers.

FIG. 3 is a cut-away perspective view of the shaft disperser of FIG. 2.

FIG. 4 is a schematic process flow diagram of an alternative method inaccordance with this invention using a pair of Bivis shaft dispersers inseries.

FIG. 5 is a generalized plot of a load/elongation curve for tissue,illustrating the determination of the MD Max Slope.

FIG. 6 is a plot of Bulk versus Panel Stiffness (stiffness as determinedby a trained sensory panel) for the bath tissues made in accordance withthis invention and commercially available creped bath tissues,illustrating the high level of bulk and low stiffness exhibited by theproducts of this invention.

FIG. 7 is a plot of Panel Stiffness versus MD Max Slope for the bathtissues made in accordance with this invention and commerciallyavailable bath tissues, illustrating the correlation of Panel Stiffnesswith the MD Max Slope.

FIG. 8 is a plot of Bulk versus MD Max Slope for the bath tissues madein accordance with this invention and commercially available bathtissues, further illustrating the high Bulk and low stiffness exhibitedby the products of this invention.

FIG. 9 is a plot similar to that of FIG. 8, but for Panel Stiffnessversus MD Stiffness Factor, illustrating the correlation of PanelStiffness and the MD Stiffness Factor.

FIG. 10 is a plot similar to that of FIG. 9, but for Bulk versus the MDStiffness Factor, further illustrating the high Bulk and low stiffnessof the products of this invention.

DETAILED DESCRIPTION OF THE INVENTION

Directing attention to the Drawing, the invention will be described infurther detail.

FIG. 1 illustrates a means for carrying out the method of thisinvention. (For simplicity, the various tensioning rolls schematicallyused to define the several fabric runs are shown but not numbered. Itwill be appreciated that variations from the apparatus and methodillustrated in FIG. 1 can be made without departing from the scope ofthe invention.) Shown is a twin wire former having a layered papermakingheadbox 10 which injects or deposits a stream 11 of an aqueoussuspension of papermaking fibers onto the forming fabric 13 which servesto support and carry the newly-formed wet web downstream in the processas the web is partially dewatered to a consistency of about 10 dryweight percent. Additional dewatering of the wet web can be carried out,such as by vacuum suction, while the wet web is supported by the formingfabric.

The wet web is then transferred from the forming fabric to a transferfabric 17 traveling at a slower speed than the forming fabric in orderto impart increased stretch into the web. Transfer is preferably carriedout with the assistance of a vacuum shoe 18 and a fixed gap or spacebetween the forming fabric and the transfer fabric or a kiss transfer toavoid compression of the wet web.

The web is then transferred from the transfer fabric to thethroughdrying fabric 19 with the aid of a vacuum transfer roll 20 or avacuum transfer shoe, optionally again using a fixed gap transfer aspreviously described. The throughdrying fabric can be traveling at aboutthe same speed or a different speed relative to the transfer fabric. Ifdesired, the throughdrying fabric can be run at a slower speed tofurther enhance stretch. Transfer is preferably carried out with vacuumassistance to ensure deformation of the sheet to conform to thethroughdrying fabric, thus yielding desired Bulk and appearance.

The level of vacuum used for the web transfers can be from about 3 toabout 15 inches of mercury (75 to about 380 millimeters of mercury),preferably about 5 inches (125 millimeters) of mercury. The vacuum shoe(negative pressure) can be supplemented or replaced by the use ofpositive pressure from the opposite side of the web to blow the web ontothe next fabric in addition to or as a replacement for sucking it ontothe next fabric with vacuum. Also, a vacuum roll or rolls can be used toreplace the vacuum shoe(s).

While supported by the throughdrying fabric, the web is final dried to aconsistency of about 94 percent or greater by the throughdryer 21 andthereafter transferred to a carrier fabric 22. The dried basesheet 23 istransported to the reel 24 using carrier fabric 22 and an optionalcarrier fabric 25. An optional pressurized turning roll 26 can be usedto facilitate transfer of the web from carrier fabric 22 to fabric 25.Suitable carrier fabrics for this purpose are Albany International 84 Mor 94 M and Asten 959 or 937, all of which are relatively smooth fabricshaving a fine pattern. Although not shown, reel calendering orsubsequent off-line calendering can be used to improve the smoothnessand softness of the basesheet.

FIG. 2 is a block flow diagram illustrating overall process steps fortreating secondary papermaking fibers in preparation for dispersing.(For virgin fibers, the fibers can be slurried with water to the desiredconsistency and introduced directly into the disperser). Shown is thepaper furnish 40 to be treated being fed to a high consistency pulper 41(Model ST6C-W, Bird Escher Wyss, Mansfield, Mass.) with the addition ofdilution water 42 to reach a consistency of about 15 percent. Prior tobeing pumped out of the pulper, the stock is diluted to a consistency ofabout 6 percent. The pulped fibers are fed to a scalping screen 43(Fiberizer Model FT-E, Bird Escher Wyss) with additional dilution waterin order to remove large contaminants. The input consistency to thescalping screen is about 4 percent. The rejects from the scalping screenare directed to waste disposal 44. The accepts from the scalping screenare fed to a high density cleaner 45 (Cyclone Model 7 inch size, BirdEscher Wyss) in order to remove heavy contaminants which have escapedthe scalping screen. The rejects from the high density cleaner aredirected to waste disposal. The accepts from the high density cleanerare fed to a fine screen 46A (Centrisorter Model 200, Bird Escher Wyss)to further remove smaller contaminants. Dilution water is added to thefine screen feed stream to achieve a feed consistency of about 2percent. Rejects from the fine screen are directed to a second finescreen 46B (Axiguard, Model 1, Bird Escher Wyss) to remove additionalcontaminants. The accepts are recycled to the feed stream to the finescreen 46A and the rejects are directed to waste disposal. The acceptsfrom the fine screen, with the addition of dilution water to reach aconsistency of about 1 percent, are then passed to a series of fourflotation cells 47, 48, 49 and 50 (Aerator Model CF1, Bird Escher Wyss)to remove ink particles and stickies. Rejects from each of the flotationcells are directed to waste disposal. The accepts from the lastflotation cell are fed to a washer 51 (Double Nip Thickener Model 100,Black Clawson Co., Middletown, Ohio) to remove very small ink particlesand increase the consistency to about 10 percent. Rejects from thewasher are directed to waste disposal. The accepts from the washer arefed to a belt press 52 (Arus-Andritz Belt Filter Press Model CPF 20inches, Andritz-Ruthner Inc., Arlington, Tex.) to reduce the watercontent to about 30 percent. Rejects from the belt press are directed towaste disposal. The resulting partially dewatered fibrous material isthen fed to a shaft disperser 53 (GR 11, Ing. S. Maule & C. S.p.A.,Torino, Italy), described in detail in FIG. 4, in order to work thefibers to improve their properties in accordance with this invention.Steam 54 is added to the disperser feed stream to elevate thetemperature of the feed material. The resulting treated fibers 55 can bedirectly used as feedstock for papermaking or otherwise further treatedas desired.

FIG. 3 is a cut-away perspective view of a preferred apparatus fortreating fibers in accordance with this invention as illustrated in FIG.2. The particular apparatus is a shaft disperser, Type GR II,manufactured by Ing. S. Maule & C. S.p.A., Torino, Italy. Shown are anupper cylindrical housing 61 and a lower cylindrical housing 62 which,when closed, enclose a rotating shaft 63 having a multiplicity of arms64. The upper housing contains two rows of knurled fingers 65 and threeinspection ports 66. At one end of the upper housing is an inlet port67. At the inlet end of the rotating shaft is driver motor 68 forturning the shaft. At the outlet end of the rotating shaft is a bearinghousing 69 which supports the rotating shaft. The inlet end of therotating shaft contains a screw feed section 70 which is positioneddirectly below the inlet and serves to urge the feed material throughthe disperser. The outlet 71 of the disperser comprises a hinged flap 72having a lever 73 which, when the disperser is closed up, is engaged byhydraulic air bags 74 mounted on the upper housing. The air bags providecontrollable resistance to the rotation of the hinged flap and henceprovide a means of controlling the back pressure within the disperser.Increasing the back pressure increases the degree to which the fibersare worked. During operation, the knurled fingers interdigitate with thearms of the rotating shaft to work the feed material therebetween.

FIG. 4 is a block flow diagram of an alternative process of thisinvention utilizing a pair of twin shaft dispersers (Bivis machines). Asillustrated, papermaking pulp, at a consistency of about 50 percent, isfed to a screw feeder. The screw feeder meters the feedstock to thefirst of two Bivis machines in series. Each Bivis machine has threecompression/expansion zones. Steam is injected into the first Bivismachine to raise the temperature of the fibers to about 212° F. (100°C.). The worked pulp is transferred to the second Bivis machineoperating at the same conditions as the first Bivis machine. The workedpulp from the second machine can be quenched by dropping it into a coldwater bath and thereafter dewatering to a suitable consistency.

FIGS. 5-10 will be discussed below in connection with the Examples.

EXAMPLES

Examples 1-20. To illustrate the invention, a number of uncrepedthroughdried tissues were produced using the method substantially asillustrated in FIG. 1. More specifically, Examples 1-19 were allthree-layered, single-ply bath tissues in which the outer layerscomprised dispersed, debonded eucalyptus fibers and the center layercomprised refined northern softwood kraft fibers. Example 20 was atwo-ply bath tissue, each ply being layered as described for theprevious examples. Cenebra eucalyptus fibers were pulped for 15 minutesat 10% consistency and dewatered to 30% consistency. The pulp was thenfed to a Maule shaft disperser as illustrated in FIG. 3. The disperserwas operated at 160° F. (70° C.) with a power input of 2.2 HPD/T (1.8kilowatt-days per tonne). Subsequent to dispersing, a softening agent(Berocell 584) was added to the pulp in the amount of 10 lb. Berocellper ton dry fiber (0.5 wight percent).

Prior to formation, the softwood fibers were pulped for 30 minutes at2.5 percent consistency, while the dispersed, debonded eucalyptus fiberswere diluted to 2 percent consistency. The overall layered sheet weightwas split 37.5%/25%/37.5% among the dispersed eucalyptus/refinedsoftwood/disperged eucalyptus layers. The center layer was refined tolevels required to achieve target strength values, while the outerlayers provided softness and bulk.

These examples employed a four-layer Beloit Concept III headbox. Therefined northern softwood kraft stock was used in the two center layersof the headbox to produce a single center layer for the three-layeredproduct described. Turbulence generating inserts recessed about threeinches (75 millimeters) from the slice and layer dividers extendingabout six inches (150 millimeters) beyond the slice were employed.Flexible lip extensions extending about six inches (150 millimeters)beyond the slice were also used, as taught in U.S. Pat. No. 5,129,988issued Jul. 14, 1992 to Farrington, Jr. entitled “Extended FlexibleHeadbox Slice With Parallel Flexible Lip Extensions and ExtendedInternal Dividers”, which is herein incorporated by reference. The netslice opening was about 0.9 inch (23 millimeters) and water flows in allfour headbox layers were comparable. The consistency of the stock fed tothe headbox was about 0.09 weight percent.

The resulting three-layered sheet was formed on a twin-wire, suctionform roll, former with forming fabrics (12 and 13 in FIG. 1) being Asten866 and Asten 856A fabrics respectively of about 64.5% and 61% voidvolume respectively. Speed of the forming fabric was 12.1 meters persecond. The newly-formed web was then dewatered to a consistency ofabout 20-27% using vacuum suction from below the forming fabric beforebeing transferred to the transfer fabric which was traveling at 9.7meters per second (25% rush transfer). Transfer fabrics employedincluded an Asten 934 and an Albany 94 M. A vacuum shoe pulling about6-15 inches (150-380 millimeters) of mercury vacuum was used to transferthe web to the transfer fabric.

The web was then transferred to a throughdrying fabric traveling at aspeed of about 9.7 meters per second. Velostar 800 and Asten934,throughdrying fabrics were used. The web was carried over aHoneycomb throughdryer operating at a temperature of about 350° F. (175°C.) and dried to a final dryness of about 94-98% consistency.

Table 1 gives more detailed descriptions of the process condition aswell as resulting tissue properties for examples 1-20, illustrating thisinvention. As used in Tables 1 and 2 below, the column headings have thefollowing meanings: “TAD Fabric” means throughdrying fabric (thedesignation “W” or “S” for the throughdrying fabric refers to which sideof the fabric is presented to the web. “W” denotes the side dominated bywarp knuckles and “S” denotes the side dominated by shute knuckles.);“#1 Trans Vac” is the vacuum used to transfer the web from the formingfabric to the transfer fabric, expressed in millimeters of mercury; “#2Trans Vac” is the vacuum used to transfer the web from the transferfabric to the throughdrying fabric, expressed in millimeters of mercury;“Cons @#1 Trans” is the consistency of the web at the point of transferfrom the forming fabric to the transfer fabric, expressed as percentsolids; “Cons @#2 Trans” is the consistency of the web at the point oftransfer from the transfer fabric to the throughdrying fabric, expressedas percent solids; “MD Tensile Strength” is the machine directiontensile strength, expressed in grams per 3 inches (7.62 centimeters) ofsample width; “MD Tensile Stretch” is the machine direction stretch,expressed as percent elongation at sample failure; “MD Max Slope” is asdefined above, expressed as kilograms per 3 inches (7.62 centimeters) ofsample width; “CD Tensile Strength” is the cross-machine tensilestrength, expressed as grams per 3 inches (7.62 centimeters) of samplewidth; “CD Tensile Stretch” is the cross-machine direction stretch,expressed as percent elongation at sample failure; “GMT” is thegeometric mean tensile strength, expressed as grams per 3 inches (7.62centimeters) of sample width; “Basis Wt” is the finished basis weight,expressed as grams per square meter; “Caliper” is the 10 sheet caliper,divided by ten, as previously described, expressed in microns; “Bulk” isthe Bulk as defined above, expressed in cubic centimeters per gram;“Panel Stiff” is the stiffness of the sheet as determined by a trainedsensory panel feeling for the relative sharpness of the folds when asheet is taken up into the hand, expressed as a number on a scale offrom 1 to 14, with higher numbers meaning greater stiffness (commercialbath tissues typically range from about 3 to about 8); and “MD StiffFactor” is the Machine Direction Stiffness Factor as defined above,expressed as (kilograms per 3 inches)-microns^(0.5).

TABLE 1 #1 #2 CONS CONS MD MD MD TRANSFER TAD TRANS TRANS #1 #2 TENSILETENSILE MAX EXAMPLE FABRIC FABRIC VAC VAC TRANS TRANS STRENGTH STRETCHSLOPE  1 ALBANY 94M W VELOSTAR 380 200 20-22 22-24 775 19.2 5.087  2ASTEM 934 W ASTEM 934 380 100 20-22 27-29 721 19.3 4.636  3 ASTEM 934 WASTEM 934 150 100 20-22 22-24 712 18.9 4.815  4 ALBANY 94M S VELOSTAR150 200 20-22 27-29 799 19.2 5.149  5 ALBANY 94M S VELOSTAR 380 10020-22 27-29 834 22.0 5.223  6 ALBANY 94M S ASTEM 934 380 100 20-22 27-29897 20.2 5.621  7 ALBANY 94M S VELOSTAR 150 100 20-22 22-24 815 19.15.543  8 ALBANY 94M W VELOSTAR 150 100 25-27 27-29 843 21.7 5.698  9ALBANY 94M W VELOSTAR 380 100 20-22 27-29 867 20.0 5.696 10 ASTEM 934 WASTEM 934 380 200 20-22 22-24 721 20.6 4.709 11 ALBANY 94M S VELOSTAR380 200 25-27 27-29 819 20.2 5.441 12 ASTEM 934 W ASTEM 934 150 20020-22 27-29 709 20.2 4.913 13 ALBANY 94M W VELOSTAR 380 200 25-27 27-29531 20.1 3.496 14 ASTEM 934 W ASTEM 934 380 200 25-27 27-29 472 19.53.244 15 ALBANY 94M S VELOSTAR 380 200 25-27 27-29 631 21.4 4.036 16ASTEM 937 S ASTEM 934 380 200 25-27 27-29 535 20.9 3.933 17 VELOSTAR800W ASTEM 934 380 200 25-27 27-29 427 16.3 3.901 18 ASTEM 934 S ASTEM934 380 200 25-27 27-29 530 21.3 4.206 19 ALBANY 94M S VELOSTAR 380 20025-27 27-29 600 20.8 4.754 20 ALBANY 94M S VELOSTAR 380 200 25-27 27-29708 18.7 5.970 CD CD MO TENSILE TENSILE BASIS PANEL STIFF STRENGTHSTRETCH GMT WT CALIPER BULK STIFF FACTOR 557 8.5 657 29.2 287  9.8 4.186 529 5.4 618 28.7 323 11.2 4.0 83 563 5.0 633 28.8 323 11.2 4.1 86 5348.2 654 28.9 305 10.5 4.6 90 629 6.9 725 30.2 305 10.1 4.7 91 632 3.9753 29.3 287  9.8 4.5 95 571 6.9 682 28.9 297 10.3 4.5 96 623 6.4 72428.7 292 10.2 4.7 97 638 7.2 744 29.7 297 10.0 4.6 98 511 5.3 607 28.3361 12.7 3.5 89 577 7.9 687 29.1 312 10.7 4.2 96 503 5.2 598 28.9 34812.0 4.0 92 428 8.3 477 20.7 249 12.0 3.5 55 324 6.0 391 19.6 315 16.03.4 58 356 11.2  474 19.8 269 13.5 3.4 66 383 5.8 453 20.1 325 16.1 3.871 306 14.8  362 19.6 330 16.8 3.4 71 299 9.4 398 19.9 335 16.8 3.2 77415 4.5 499 20.0 287 14.3 3.8 81 494 8.6 591 38.0 388 10.1 3.2 83

Referring now to FIGS. 5-10, various aspects of the invention will bedescribed in further detail.

FIG. 5 is a generalized load/elongation curve for a tissue sheet,illustrating the determination of the MD Max Slope. As shown, two pointsP1 and P2, the distance between which is exaggerated for purposes ofillustration, are selected that lie along the load/elongation curve. Thetensile tester is programmed (GAP [General Applications Program],version 2.5, Systems Integration Technology Inc., Stoughton, Mass.; adivision of MTS Systems Corporation, Research Triangle Park, N.C.) suchthat it calculates a linear regression for the points that are sampledfrom P1 to P2. This calculation is done repeatedly over the curve byadjusting the points P1 and P2 in a regular fashion along the curve(hereinafter described). The highest value of these calculations is theMax Slope and, when performed on the machine direction of the specimen,is called the MD Max Slope.

The tensile tester program should be set up such that five hundredpoints such as P1 and P2 are taken over a two and one-half inch (63.5mm) span of elongation. This provides a sufficient number of points toexceed essentially any practical elongation of the specimen. With a teninch per minute (254 mm/min) crosshead speed, this translates into apoint every 0.030 seconds. The program calculates slopes among thesepoints by setting the 10th point as the initial point (for example P1),counting thirty points to the 40th point (for example, P2) andperforming a linear regression on those thirty points. It stores theslope from this regression in an array. The program then counts up tenpoints to the 20th point (which becomes P1) and repeats the procedureagain (counting thirty points to what would be the 50th point (whichbecomes P2), calculating that slope and also storing it in the array).This process continues for the entire elongation of the sheet. The MaxSlope is then chosen as the highest value from this array. The units ofMax Slope are kg per three-inch specimen width. (Strain is, of course,dimensionless since the length of elongation is divided by the length ofthe jaw span. This calculation is taken into account by the testingmachine program.)

FIG. 6 is a plot of Bulk versus Panel Stiffness for bath tissues made inaccordance with this invention (Examples 1-20 plotted as points a-t,respectively) and for a number of commercially available creped bathtissues plotted as either a “1” representing a single-ply product, a “2”representing a two-ply product and a “3” representing a three-plyproduct. This plot illustrates the unique combination of high Bulk andlow stiffness possessed by the products of this invention.

FIG. 7 is a plot of Panel Stiffness versus MD Max Slope for the sameproducts, illustrating the correlation of MD Max Slope with stiffness asmeasured by a trained sensory panel. This plot shows that MD Max Slopeis an objective measure of panel stiffness.

FIG. 8 is a plot of Bulk versus MD Max Slope for the same products,illustrating the combination of high Bulk and low stiffness (as measuredby the MD Max Slope) exhibited by the products of this invention.

FIG. 9 is a plot similar to the plot of FIG. 7, but Panel Stiffness isplotted against the MD Stiffness Factor instead of MD Max Slope,illustrating that the MD Stiffness Factor is also a valid measure ofstiffness.

FIG. 10 is a plot similar to the plot of FIG. 8 with Bulk plotted versusthe MD Stiffness Factor, illustrating the combination of high Bulk andlow stiffness (as measured by the MD Stiffness Factor) exhibited by theproducts of this invention.

It will be appreciated that the foregoing examples, given for purposesof illustration, are not to be construed as limiting the scope of thisinvention, which is defined by the following claims and all equivalentsthereto.

We claim:
 1. A soft tissue product comprising one or more tissue pliesand having a Bulk of 9 cubic centimeters or greater per gram, a machinedirection stretch of 10 percent or greater and a MD Max Slope of 13 orless, said product having a strength providing layer and one or morerelatively weaker layers in which the relatively weaker layer(s) hasincreased bulk compared with the strength providing layer.
 2. Theproduct of claim 1 in which one or more of the relatively weakerlayer(s) is formed by fibers which have been chemically or mechanicallymodified to increase softness.
 3. The product of claim 2 comprisingcurled fibers in the relatively weaker layer(s).
 4. The product of claim3 in which the curled fibers are dispersed fibers.
 5. The product ofclaim 1 in which the strength providing layer comprises softwood fibers.6. The product of claim 1 in which one or more of the relatively weakerlayer(s) is formed by hardwood eucalyptus fibers.
 7. The product ofclaim 1 having a MD Max Slope of 10 or less.
 8. The product of claim 1having a MD Stiffness Factor of 100 or less.
 9. The product of claim 1having a Bulk of from 9 to 20 cubic centmeters per gram.
 10. The productof claim 1 having a Bulk of from 10 to 15 cubic centimeters per gram.11. The product claim of 1 having a MD Max Slope of 5 or less.
 12. Theproduct of claim 1 having a MD Max Slope of from 3 to
 6. 13. The productof claim 1 having a MD Stiffness Factor of from 50 to
 100. 14. Theproduct of claim 1 having a machine direction stretch of from 15 to 25percent.
 15. The product of claim 1 having one uncreped throughdriedply.
 16. The product of claim 1 having two uncreped throughdried plies.17. The product of claim 1 having three uncreped throughdried plies. 18.The product of claim 1 having four uncreped throughdried plies.
 19. Theproduct of claim 1 wherein said plies have two or more layers, at leastone layer being an outwardly facing layer and comprising 20 percent ormore curled fibers.
 20. The product of claim 1 wherein said plies havetwo or more layers, at least one layer being an outwardly facing layerand comprising 80 percent or more curled fibers.